Electrorecovery of gold and silver from thiosulphate solutions

ABSTRACT

The present invention is related to the mining industry and treatment of mineral and materials that contain gold and silver. Specifically, it is related to a process to recover gold and silver, from copper thiosulfate solutions with a autogenerated electrolysis process. The advantages of the present invention, relative to those of the state of the technique, reside in the increased velocity compared with cementation using copper, without employing electric current. Our process is characterized by operating in an electrochemical autogeneration cell, in which the anode and cathode are connected in short circuit and the anodic and cathodic compartments are separated by an ion exchange membrane. Additionally, using a copper anode and the stripped solution as the anolyte, the levels of soluble copper are maintained stable, conserving the leaching power of the thiosulfate solutions, whereby it is possible to recycle them back to the leaching stage.

FIELD OF THE INVENTION

The present invention is related to the mining industry and treatment ofmineral and materials that contain gold and silver. Specifically, it isrelated to a process to recover gold and silver, from copper thiosulfatesolutions with a autogenerated electrolysis process, in which themetallic values are recovered from the rich solution in the cathodiccompartment. The barren solution is then used as the anolyte,re-establishing the copper concentration needed to be recycled back tothe leaching stage.

BACKGROUND OF THE INVENTION

At present, gold and silver are obtained from their minerals,concentrates and other materials, using different processes. Theseprocesses are in function of the nature of the gold and silvercontaining material, as well as their grade. Accordingly, if it is ahigh grade material, smelting is employed. On the other hand, if thematerial contains only small amounts of gold and silver, ahydrometallurgical treatment is usually selected (leaching).

Since the end of the XIX century, the process based on cyanidesolutions, has been successfully used for leaching gold and silver fromlow grade materials. However, cyanide solutions are highly toxic.Additionally, some materials are refractory towards this process orcontain a high copper concentration, which consumes large amounts ofcyanide during leaching, as the following article teaches [G.Senanayake, Gold leaching in non-cyanide lixiviant systems: criticalissues on fundamentals and applications, Mineral Engineering2004(17)785-201].

Several alternatives to cyanidation have been proposed, among them isthe method based on thiosulfate. This chemical system has been utilized,on a pre-industrial scale, since the 1920's [Fathi Habashi. A Textbookof Hydrometallurgy, 2nd edition (Second ed). Quebec City, Canada:Métallurgie Extractive Québec, 1999]. However, the elevated reagentconsumption, caused by its oxidation to tetrathionate and even sulfateby the cupric ions (Cu(II)), has hindered its large scaleimplementation.

Recently, this inconvenience has been solved with additives that modifythe oxidative properties of the cupric ions, [Gretchen Lapidus-Lavine,Alejandro Rafael Alonso-Gómez, José Angel Cervantes-Escamilla, PatriciaMendoza-Münoz and Mario Francisco Ortiz-Garcia, “Mejora al Proceso deLixiviación de Plata de Soluciones de Tiosulfato de Cobre (Improvementto the Silver Leaching Process with Copper Thiosulfate Solutions)”],Mexican patent granted the 26 Feb. 2008, MX 257151], by limitingthiosulfate consumption to less than 5% of its initial value[Alonso-Gómez, A. R. and Lapidus, G. T. (2009), “Inhibition of LeadSolubilization during the Leaching of Gold and Silver in AmmoniacalThiosulfate Solutions (effect of phosphate addition)”, Hydrometallurgy,99(1-2), 89-96].

On the other hand, the recovery of values from the thiosulfate baths hasbeen performed principally by cementation, in which a reducing agent,usually a metal, is added to generate a redox reaction which producesgold and silver in their metallic state. A disadvantage of thistechnique is that it is not possible to adequately control the reductivecapacity of the agent, which causes a poor separation efficiency,obtaining gold and silver contaminated with copper.

Direct electrodeposition, used as a separation method, is a viableoption, including from solutions with low concentrations of gold andsilver, even when the copper ion concentration is more than 50 timesgreater than that of silver and over 100 times that of gold[Alonso-Gómez, A. R., Lapidus, G. T. and González, I., “Proceso deLixiviación y Recuperación de Plata y Oro con Soluciones de TiosulfatoAmoniacales de Cobre, solicitud PCT/MX2009/000022, fecha 14 Mar. 2008(WO20097113842, publicada 17 Sep. 2009)]. To attain efficiencies greaterthan 50%, a rotating cylinder electrode was employed in a reactor withseparate anodic and cathodic compartments in order prevent the oxidationof the thiosulfate and the re-oxidation of the deposited gold andsilver. In this type of cell, deposits were obtained with less than 2%impurities [Alonso, A. R., Lapidus, G. T. and González, I. (2008),“Selective silver electroseparation from ammoniacal thiosulfate leachingsolutions using a Rotating Cylinder Electrode reactor (RCE)”,Hydrometallurgy, 92 (3-4), 115-123].

Despite the excellent results obtained with this type of reactor, therelatively low current efficiencies can be considered a disadvantage dueto the high cost of electricity.

Recently, autogenerated electrolyses have been explored. These consistof a two electrode cell, in which metal ions are reduced and depositedon the cathode, differing from a traditional current-drivenelectrodeposition, in the fact that the anode is made of a materialwhose oxidation potential is less than the reductive potential of themetal ions and therefore does not require addition electricity to drivethe process. Upon anode oxidation, an electron flow travels through anelectrical conductor to the cathode, where the electrodeposit occurs.For this reason, the anodic and cathodic compartment must be separatedby an ion exchange membrane.

Autogenerated electrolysis shares with cementation the principle thatthe oxidation of a metal is used to reduce another more noble.

However, in autogenerated electrolysis, the separated anodic andcathodic compartments allow, on one hand, the election of the substrateupon which the metal is deposited (similar to a conventionalelectrolysis), eliminated the contamination of the deposit. On theother, because the anode is in contact with a solution which isdifferent from the one that contains the metallic ions to be deposited,it is also possible to tailor the anolyte composition according to therequirement of the process and in this manner modulate the reductivepower of the system.

This procedure is adequate for gold and silver recovery from thiosulfatesolutions, eliminating the need for electrical energy through theoxidation of a metallic anode. It is important to mention that theelection of the anode material will depend on the difference between theredox potentials of the anode and cathode, as well as the advantagesthat the dissolution of a certain material might offer to the process.This should result in lower process costs.

OBJECTIVES OF THE INVENTION

One objective of the present invention is to provide a selectiveseparation process for gold and silver from thiosulfate solutions, at anincreased velocity compared with copper cementation, without the use ofelectrical current.

Another objective is to accomplish the aforementioned task using thebarren solution as the anolyte, conserving in this manner the level ofsoluble copper, in order to maintain the composition of the thiosulfatesolution so that it can be recycled back to the leaching stage.

Other objectives and advantages that apply the principles and arederived from the present invention may be apparent from the study of thefollowing description and diagrams that are included here forillustrative and not limitative purposes.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is designed to solve the problem of gold andsilver separation from copper thiosulfate leaching solutions, providingan improvement over the traditional separation methods (cementation andexternal current-driven electrolysis). This improvement is characterizedby the use of a novel autogenerated electrolysis process, employing acommercial copper sheet as the anode and a titanium cathode, in areactor with anodic and cathodic compartments separated by ion exchangemembrane which prevents the contamination of the thiosulfate solution.

The membrane achieves the purpose of separating the anodic and cathodicsections, to prevent the solutions used in each compartimentsto frommixing. This is important to avoid cementation of gold and silver on thecopper surface, which slows the process and contaminates the product. Onthe other hand, it is important to consider that the rich (pregnant)solution (located in the cathodic compartment) is poor in copper due tothe nature of the leaching process, and its oxidation power is limited;this allows an efficient gold and silver deposition because there islittle re-dissolution. By preventing contact of the pregnant solutionwith the copper anode, the copper concentration in this solution is keptlow and for this reason the membrane plays a double role.

In order to better understand the characteristics of the invention, thefollowing description is accompanied by diagrams and figures, which forman integral part of the same and are meant to be illustrative but notlimitative and are described in the following section.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the process for electrodepositing goldand silver in an electrochemical autogeneration cell.

FIG. 2 shows a schematic diagram of the electrochemical autogenerationcell

FIG. 3 corresponds to a graph indicating the change in the silverconcentration during the electrolysis performed in Example 1.

FIG. 4 is a diagram of the recirculation process of lots A and B of theleaching solution, used in Example 2.

FIG. 5 shows a graphic representation of the silver concentration changeduring the first leach LA1, performed on lot A (solid lines andmarkers), as well as during the first autogenerated electrolysis Ca1(dotted line, hollow markers).

FIG. 6 is a series of graphs that compare the quantity of silverremaining in solution throughout the electrolyses Ca1, Ca3 and Ca5performed on lot A (markers ∘, □ and Δ, respectively).

FIG. 7 shows the lead concentration during the first leach LA1 performedon lot A.

DETAILED DESCRIPTION OF THE INVENTION

The process referred to in the present is performed according to theillustration in FIG. 1:

-   -   An ammoniacal thiosulfate solution pregnant with gold and silver        ions, originating from the leaching stage (100) and after having        been filtered (200), is introduced into the cathodic compartment        (310) of the electrochemical reactor (300).    -   The electrochemical reactor possesses and ion exchange membrane        (350) that separates the cathodic (310) and anodic (320)        compartments.    -   A solution, stripped of the precious metals (360), is introduced        into the anodic compartment of the electrochemical reactor        (320).    -   The cathode (330) and anode (340) are connected in short circuit        (360)    -   The solutions in the cathodic and anodic compartments (310 and        320) are stirred during the entire time of the        electro-deposition process that could range from ½ to 4 hours.    -   Once the electrodeposition process has finished, the cathode        (330) is removed from the reactor and mechanically scraped to        obtain the gold and silver metals. The solution in the cathodic        compartment is placed in the anodic compartment, ready for the        next electrodeposition cycle (360).    -   The solution in the anodic compartment (320), having been        enriched with the necessary reagents (copper ions), is recycled        back to the leaching stage (140).    -   The leaching reactor is charged with fresh leachable solid        material (160). Fresh solution (150) is only fed to the leaching        reactor in the initial cycle.

The operation of the electrochemical autogeneration reactor isrepresented in FIG. 2:

-   -   The reactor consists a single preferentially rectangular        reservoir (400), although it is not limited to said        configuration.    -   The reactor is divided in at least two compartments, although it        may have more.    -   The compartments are divided by a cationic membrane (450) that        hinders the passage of silver-thiosulfate and gold thiosulfate        ions from the cathodic (410) to the anodic (430) compartment.    -   The cathode (420) can be a titanium sheet or screen.    -   The anode (440) is a copper sheet.    -   Once the solutions are charged to the reactor, the electrodes        are connected in short circuit (460).    -   The solutions are mechanically stirred (470) during the        electrodeposition time.    -   The cathode should be mechanically treated to remove the gold        and silver deposit.    -   The anode should be changed periodically, since it is consumed        during the electrodeposition process.

EXAMPLES Example 1

To better understand the invention, one of the many experiments isdetailed as an example, which employs a reactor such as that schematizedin FIG. 2. A 60 cm² (exposed geometrical area) titanium plate was usedas the cathode and a copper plate with the same exposed area was theanode. A synthetic solution, prepared with the composition that appearsin Table 1, which simulates real solutions after the leaching stage, wasintroduced into the cathodic compartment (410).

TABLE 1 Composition of the solution used in the cathodic compartment ofthe autogeneration electrolytic reactor. Component Composition (mol/L)Ag(I) 1 × 10⁻³ Na₂S₂O₃ 0.2 CuSO₄ 0.05 EDTA⁴⁻ 0.025 (NH₄)₂(HPO₄) 0.1 NH₃0.6

A synthetic solution, poor in copper ions, whose composition is detailedin Table 2, was placed in the anodic compartment (430).

TABLE 2 Composition of the solution used in the anodic compartment ofthe autogeneration electrolytic reactor Component Composition (mol/L)Na₂S₂O₃ 0.2 CuSO₄ 0.025 EDTA⁴ 0.025 (NH₄)₂(HPO₄) 0.1 NH₃ 0.6

The solutions were prepared with analytical grade reagents and deionizedwater (1×10¹⁰ MΩcm⁻¹). Once the solutions were placed in theirrespective compartments, the electrodes were connected in short circuit.Stirring in both compartments was maintained during theelectrodeposition process. Samples of the solution were taken every 20minutes for four hours, after which time the test was detained. Thesamples were analyzed for silver and copper with atomic absorptionspectrometry.

In FIG. 3, a graphic representation is shown of results of theelectrodeposition process, performed in the reactor of FIG. 2. Thedecrease in silver concentration is constant from the beginning of theelectrolysis, attaining 50% of its initial value after only 60 minutes.Subsequently, the descent is slower, typical of first order reactionkinetics in a batch reactor, reaching 4% after 4 hours.

On the other hand, the copper concentration in the cathodic compartmentremained constant during the electrolysis (data not shown), indicativeof a selective silver deposit.

In order to determine the leaching power of the recycled solution, afterhaving stripped the silver ions in the autogeneration process,experiments were performed with real leaching solutions, whose resultsare shown in the following example.

Example 2

As was shown in FIG. 1, the recirculation scheme used in the presentinvention employs two lots of the thiosulfate leaching solution, whichare alternated in each one of the reactor compartments (FIG. 2), as wasmentioned in the Detailed Description section. The same reactor was usedas in Example 1, with a copper sheet as the anode and a titanium sheetas the cathode, both with an exposed geometric area of 60 cm².

To better understand the process, a block diagram is shown (FIG. 4), inwhich the passage through the process of lots A and B of the leachingsolution are shown, without the solid streams. By observing only lot A(solid lines), stream Al enters the first leach (LA1), and afterseparating out and discarding the solid residue, stream A2 (pregnantsolution) enters the cathodic compartment (Ca1) of the electrolyticreactor, where the silver electrodeposition takes place; only in thisstage of the process is synthetic solution (Stream S1) used in theanodic compartment (An1).

Stream A3, stripped of its values, is placed in the anodic compartmentof the reactor (An2), where the first electrodeposit from the pregnantsolution lot B (Ca2) occurs.

Stream A4 is sent back to a new leaching stage (LA2), where it iscontacted with fresh mineral. The pregnant solution (A5) is sent to theelectrochemical reactor for silver recovery in the cathodic compartment(Ca3). In this case, the anodic compartment is occupied by the solutionof lot B originating from Ca2.

Subsequently, the process is repeated, passing the stream A6 to theanodic compartment (An4) during the electrodeposition of B5 (Ca4).

Finally, stream A7 is again introduced into the leaching stage withfresh mineral, obtaining a pregnant solution in stream A8.

The route that lot B follows is practically the same as lot A. Table 3shows the initial composition used in the leaching solutions for bothlots; the volume of each one was 250 mL. Each leach used 2.5 g of a leadconcentrate from Fresnillo mine, whose silver content is 24 kg/ton withapproximately 25% of lead.

TABLE 3 Composition of the leaching solutions used in lots A and B.Component Composition (mol/L) Na₂S₂O₃ 0.2 CuSO₄ 0.05 EDTA⁴ 0.025(NH₄)₂(HPO₄) 0.1 NH₃ 0.6

In FIG. 5 the silver concentration during the first leach is shown(solid lines and markers), as well as the electrodeposition process inthe cathodic compartment Ca1 (dashed line and hollow markers). It isimportant to consider that the silver content in this mineral is veryhigh, explaining the reason for extractions above 200 ppm, a value closeto the solubility limit for this metal ion in thiosulfate solutions.These high values of silver in solution are the reason that the silverconcentration only decreases to 50% of its original value in theautogenerated electrolysis (Ca1). Additionally, because of the highdissolved lead concentration (200 ppm), there is competition with thesilver in the electrodeposition process. This could represent anenormous loss in the traditional cyanidation process; however, in thiscase, the thiosulfate solution is recycled back to the leaching stage,the gold and silver remaining in solution are separated in subsequentcycles.

In the subsequent leaches performed with lot A, within the recirculationscheme, extractions similar to that observed in LA1 were achieved(approximately 200 ppm silver ions). The results obtained in leacheswith lot B are very similar, again observing the solubility limitationof 200 ppm Ag(I). These results are significant since they show that thethiosulfate solution maintains its leaching power after three cycles ofleaching-electrodeposition, in which no additional reagent was added tomake-up the solutions.

Finally, a comparison of the change in silver concentration during theautogenerated electrolyses for lot A is shown in FIG. 6. Theelectrolyses Ca3 and Ca5 present similar behavior to that registered forCa1 (first electrolysis of lot A). The quantity of silver that remainsafter the electrolyses Ca2 and Ca3 is similar, indicating that there isno accumulation of silver ions in the recycling process; in other words,the silver extracted in the leach is separated in the autogeneratedelectrolysis stage. The behavior of lot B during the electrolyses (datanot shown here) is practically the same exhibited by lot A.

It is important to remember that the mineral leached was a leadconcentrate, the reason for which an important quantity of this metaldissolved, despite the use of phosphate to inhibit this process. In FIG.7, the lead concentration is shown during the first leach of lot A,where it can be appreciated that the concentration of Pb(II) is similarto that of silver. Also, in the corresponding electrolysis, the leadconcentration decreases approximately 35% during the first 20 minutes.This competition (inexistent in Example 1 with the synthetic solution)could be the cause that the silver recovery did not exceed 60%.Additionally, it must be considered that treating such high grade silverminerals would originate solubility problems during leaching, as well aselectrode saturation in the electrodeposition stage. In these cases, itis possible to increase the thiosulfate concentration to increase thesolubility of the Ag(S₂O₃)₂ ³⁻ complex, even though a largerelectroactive area for the cathode would be required.

In any event, these examples are evidence that the use of aautogenerated electrolysis reactor is viable within aleaching-electroseparation scheme, maintaining the leaching capacity ofthe thiosulfate solution.

1.-3. (canceled)
 4. An autogenerated electrolysis cell for theelectrorecovery of silver from thiosulfate leaching solutions comprisinga cathodic and an anodic compartment separated by a ion exchangemembrane, a copper anode and a titanium cathode, connected in shortcircuit, a catholyte consistent in a pregnant leaching solution and ananolyte, stripped of its gold and silver in the cathodic compartment. 5.A silver electrorecovery process from thiosulfate leaching solutions, inan autogenerated electrolysis cell, consisting of the following: in thefirst stage, feed the cathodic compartment with a solution proceedingfrom the leaching step, feed to the anodic compartment a syntheticsolution similar to the catholyte, but without dissolved silver,maintain the operation of the electrolysis cell during a specified time,mechanically recover the silver deposit by being performed in anautogenerated electrolysis cell as was stated in the previous claim; thepredetermined time that the cell operates is that which permits thesilver concentration in the catholyte and the copper concentration inthe anolyte to achieve a predetermined level; successive stages followthe first, in which the anolyte is the catholyte after having beenstripped of silver and the leaching solution is the anolyte after havingbeen enriched with copper ions.
 6. The silver electrorecovery fromthiosulfate leach solutions of claim 5, wherein the predetermined levelof the copper concentration is adequate, on one hand, to maintain thepotential difference necessary from 4 to 7 g/L of dissolved copper forthe autogenerated electrodeposit and, on the other, for silver leaching.